Flexible printed circuit (fpc) board

ABSTRACT

A flexible printed circuit (FPC) board having reinforcing pattern against bending is disclosed. The FPC board provides an RF interconnection extending from an RF electrode. Two ground electrodes are formed in respective sides of the RF electrode. The ground electrodes extend respective extended portions along the RF interconnection to protect the RF interconnection from breakage due to bending of the FPC board. The extended portion provides an end portion bent toward the RF interconnection to compensate for impedance mismatching of the RF interconnection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an arrangement of a flexible printed circuit (FPC) board.

2. Background Arts

An FPC board has been often used to connect an electrical or optical module with a printed circuit board. Recent modules may operate at a speed reaching or sometimes exceeding 10 Gbps and an FPC board is inevitable to carry such high speed signals thereon. In order to transmit a high speed signal as keeping quality thereof, a transmission line often takes an arrangement of the micro-strip line and/or the co-planar line. When the FPC board arranges the micro-strip line and/or the co-planar line, a line or an interconnection on the FPC board often becomes slim enough. Such a line is easily broken due to a bend of an FPC board.

SUMMARY OF THE INVENTION

An aspect of the present invention relates to an FPC board that is to be soldered to a rigid circuit board. The FPC board comprises a top electrode provided in the top surface of one end of the FPC board; and a back electrode provided in the back surface of the one end of the FPC board. The top electrode is electrically connected to the back electrode through a via hole. The back electrode is to be soldered to a pad provided in the rigid circuit board. A feature of the FPC board is that at least one of the top electrode and the back electrode extends an interconnection, while, another of the top electrode and the back electrode extends an extended portion that is terminated in the one end of the FPC board.

Another FPC board of the present application includes a top RF electrode, a back RF electrode, a top ground electrode, and a back ground electrode. The top RF electrode and the top ground electrode are provided in one end of a top surface of the FPC board. The back RF electrode and the back ground electrode are provided in the one end of a back surface of the FPC board and soldered to the rigid circuit board. The top ground electrode is provided next to the top RF electrode and the back ground electrode is provided next to the back RF electrode. The top RF electrode is electrically connected to the back RF electrode through a via hole, while, the top ground electrode is electrically connected to the back ground electrode through a via hole. The back ground electrode extends a ground pattern as forming a gap to the back RF electrode. A feature of the FPC board is that the top RF electrode extends an RF interconnection; while, the top ground electrode extends an extended portion terminated in the one end of the FPC board.

Still another FPC board of the present application includes top RF electrodes and top ground electrodes both provided in one end of the top surface of the FPC board, and back RF electrodes and back ground electrodes both provided in the one end of a back surface of the FPC board. The top RF electrodes are electrically connected to the back RF electrodes through via holes, and the top ground electrodes are electrically connected to the back ground electrodes through via holes. The top RF electrodes and the top ground electrodes are arranged alternately, and the back RF electrodes and the back ground electrodes are arranged alternately. The back RF electrodes and the back ground electrodes are to be soldered to the rigid circuit board. The back ground electrodes are electrically connected through a ground pattern with gaps against the back RF electrodes. A feature of the FPC board is that the top RF electrodes extend RF interconnections, while, the top ground electrodes each has an extended portion with a width substantially equal to widths of the top ground electrodes and terminated in the one end of the FPC board.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIG. 1A is a plan view of a top surface of a flexible printed circuit (FPC) board according to the first embodiment of the present application, and FIG. 1B is a plan view of a back surface of the FPC board shown in FIG. 1A;

FIG. 2 shows a cross section of the FPC board soldered to a rigid circuit board;

FIG. 3A is a plan view of a top surface of another FPC board according to the second embodiment, and FIG. 3B is a plan view of a top surface of still another FPC board according to the third embodiment of the present application;

FIG. 4A is a plan view of a top surface of an FPC board according to the fourth embodiment, and FIG. 4B is a plan view of an FPC board according to the fifth embodiment of the present application;

FIG. 5A is a plan view of a top surface of an FPC board according to the sixth embodiment, and FIG. 5B is a plan view of an FPC board according to the seventh embodiment of the present application; and

FIG. 6A is a plan view of a top surface of an FPC board according to the eighth embodiment of the present application, and FIG. 6B is a plan view of a top surface of an FPC board modified from the FPC board shown in FIG. 6A.

DESCRIPTION OF EMBODIMENTS

Next, some embodiments according to the present invention will be described as referring to drawings. In the explanation of the drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without duplicating explanations.

First Embodiment

FIGS. 1A and 1B are plan views of a flexible printed circuit (FPC) board 1 according to the first embodiment of the present invention, where FIG. 1A illustrates a top surface and FIG. 1B shows a back surface thereof. The FPC board 1 shown in FIGS. 1A and 1B is implemented within an optical transceiver; specifically, the FPC board 1 is prepared independently for a transmitter optical subassembly and a receiver optical subassembly to electrically connect those subassemblies with a printed circuit board.

The FPC board 1 provides the top and bottom surfaces, 2 c and 2 d, respectively, and two ends, 2 a and 2 b. The FPC board 1 further provides, in the end portion A, a plurality of electrodes (top electrodes), 3 a to 3 g, interconnections 4 extending from respective outer most electrodes, 3 a and 3 g, and an RF (Radio Frequency) interconnection 5 connected to the center electrode 3 d, which will be called as the RF electrode. Moreover, the FPC board 1 in the back surface 2 d thereof also provides electrodes (back electrodes), 3 a to 3 g, interconnections 6 extending from the electrodes 3 b and 3 f, and a ground (GND) pattern 7 connected to the electrodes 3 c and 3 e. The other end portion opposite to the end portion A also provides seven (7) electrodes corresponding to the electrodes, 3 a to 3 g, in the end portion A. The electrodes, 3 a to 3 g, in the top surface 2 c are electrically connected to electrodes, 3 a to 3 g, in the back surface 2 d through respective via holes 3 v.

The FPC board 1 of the present embodiment shows a line symmetry with respect to an axis Ax passing from a center of the first edge 2 a to the second edge 2 b. The first and second edges, 2 a and 2 b, each extends straight and intersects the axis Ax in a right angle. The FPC board 1, as illustrated in FIGS. 1A and 1B, provides a narrowed mid body between two widened end portions A, and joining portions 2 e having a width gradually increasing from the interface to the mid body to the other interface to the end portion A.

The electrodes, 3 a to 3 g, which are made of metals, provide via holes 3 v electrically connecting portions in the top surface 2 c to other portions in the back surface 2 d. The electrodes, 3 a to 3 g, in the back surface 2 d thereof are to be soldered to respective pads provided in the rigid circuit board 10.

Referring to FIG. 2, the FPC board 1 is soldered to the rigid substrate 10 such that the boundary L of the FPC board 1, which distinguishes the electrodes, 3 a to 3 g, from the interconnections, 4 and 6, the RF interconnection 5, and the ground pattern 7, coincides with the edge 10 b of the pad 10 a. Specifically, referring to FIGS. 1A and 1B again, the end portion A may be divided in two regions, A₁ and A₂, one of which A₂ provides the electrodes, 3 a to 3 g, while, the other A₂ set inward from the former region A₁provides extended electrodes, 3 c and 3 e. The former region A₂ may be called as the electrode region and the latter is called as the extended region. The electrodes, 3 a to 3 g, extend inward from the edge 2 a as intersecting the edge 2 a in a right angle; that is, the edges of the electrodes, 3 a to 3 g, coincide with the edge 2 a of the FPC board 1, and the other edge of the electrodes, 3 a, 3 b, 3 d, 3 f, and 3 g, align with the boundary line L. The boundary line L virtually connects the boundary C₁ between the electrodes, 3 a and 3 g, and the interconnection 4 to the boundary C₂ between the electrode 3 d and the RF interconnection 5. On the other hand, the inner edges of the electrodes, 3 c and 3 e, also extend inward as crossing the boundary line L.

The interconnection 4 extend, in the top surface 2 c of the FPC board 1, from the ends C₁ of the electrodes, 3 a and 3 g, and run in parallel with edges of the joining portion 2 e. The RF interconnection 5 extends straight in the top surface 2 c from the end C₂ of the electrode 3 d along the center axis Ax. Because portions in the back surface 2 d corresponding to the RF interconnection 5 provide the ground pattern, which emulates the micro-strip transmission line, the RF interconnection 5 inherently has characteristic transmission impedance capable of transmitting high frequency signals reaching and sometimes exceeding 10 Gbps between the optical subassemblies and the rigid circuit board 10. In order to show the characteristic transmission impedance, the RF interconnection 5 has a width narrower than a width of the interconnections, 4 and 6.

In the back surface 2 d, the end of the electrode 3 d forms a gap 2 f to the ground pattern connected to the electrodes, 3 c and 3 e, in respective sides of the center electrode 3 d. However, the RF interconnection 5 in the top surface 2 c in the portion corresponding to the gap 2 f has the width substantially same with that in the mid body. Accordingly, the RF interconnection 5, in particular, the portion corresponding to the gap shows the characteristic impedance slightly greater than that in the mid body. This impedance mismatching may degrade quality of high frequency signals carried on the RF interconnection 5.

The FPC board 1 further provides another interconnection 6 extending inward from the boundary C₃ against the electrodes, 3 b and 3 f, in the back surface 2 d. The interconnection 6 also runs substantially in parallel to the edges of the joining portion 2 e and to the edge of the ground pattern in the mid body. The ground (GND) pattern 7, as described above, extend from the electrodes, 3 c and 3 e, in respective sides of the center electrode 3 d and combined in the extended region A₁ to occupy a large part of the back surface 2 d of the mid body, in particular, the whole center portion of the back surface 2 d corresponding to the RF interconnection 5 in the top surface 2 c. The center pad 3 d connected to the RF interconnection 5 and two electrodes, 3 c and 3 e, in respective sides of the center electrode 3 d imitates the co-planar transmission line which may partly compensate the impedance mismatching caused in the RF interconnection 5 in the part of the gap 2 f. The interconnections, 4 and 6, with widths wider than that of the RF interconnection 5 may carry DC signals or signals having low frequency components. Those signals are unconcerned with the transmission impedance.

The FPC board 1 of the present embodiment provides in the electrodes, 3 c and 3 e, respective extended portions, 3 q and 3 r, extending straight within the extended region A₁ as keeping a width thereof from the edge 2 a. Although the extended portion, 3 q and 3 r, are set in respective sides of the RF interconnection 5, the extended portions, 3 q and 3 r, do not affect the characteristic impedance of the RF connection 5. The characteristic impedance of the RF interconnection 5 may be primarily determined by the width thereof and the ground pattern 7 in the back surface 2 d. The function of the extended portions, 3 q and 3 r, are to reinforce the FPC board 1 against the bending, that is, to protect the RF interconnection 5 from a breakage by the bending of the FPC board 1.

Referring to FIG. 2 again, when the FPC board 1 is connected to the rigid circuit board 10, the all electrodes, 3 a to 3 g, are soldered to the pads 10 a on the circuit board 10, and the FPC board 1 is often bent upward to connect the electrodes, 3 a to 3 g, in the other end 2 b to the optical modules. Under such an arrangement, the FPC board 1 is likely to be bent along the boundary line L by a small radius, which often causes breakages of the interconnections, 4 to 6, in particular the RF interconnection 5 because of the narrowed line width thereof.

The extended portions, 3 q and 3 r, which extend from the electrodes, 3 c and 3 e, to the joining portion 2 e, that is, the ends P of the extended portions, 3 q and 3 r, exist within the joining portion 2 e, as keeping the widened width thereof may effectively prevent the FPC board 1 from being bent along the boundary line L or bent in the extended region A₁ without influencing the electrical characteristics of the RF interconnection 5.

The FPC board 1 of the present embodiment provides the extended portions only in the electrodes, 3 c and 3 e, which are electrically connected to the ground pattern 7. The FPC board 1 may provide other extended portions in the electrodes, 3 b and 3 f, in the top surface 2 c, and in the electrodes, 3 g and 3 a, in the back surface 2 d. The former electrodes, 3 b and 3 f, are electrically connected to the interconnections 6 in the back surface 2 d, while, the latter electrodes, 3 a and 3 g, are connected to the other interconnections 4 in the top surface 2 c. These interconnections, 4 and 6, carry DC signals and/or signals with lower frequency components; accordingly, the extended portions to be provided in those electrodes, 3 a, 3 b, 3 f and 3 g, may not influence the electrical performance of the FPC board 1. Those extended portions may effectively suppress the FPC board 1 from being bent along the boundary line L when the electrodes, 3 a to 3 g, in the back surface 2 d thereof are soldered to the pads 10 a on the rigid board.

Second Embodiment

FIGS. 3A and 3B show top and back surfaces of an FPC board 1A according to the second embodiment of the present invention. The FPC board 1A shown in FIGS. 3A and 3B is an example that all electrodes, 3 a to 3 c and 3 e to 3 g, provided for DC signals including the ground and/or signals with low frequency components have respective extended portions in the extended regions A₁ and the joining portion 2 e. One feature of the present embodiment is that the extended portions are bent inward along the edge of the joining portion 2 e.

Specifically, the electrodes, 3 c and 3 e, put in respective sides of the RF electrode 3 d have the end portions, 3 j and 3 k, extending from the edge 2 a as maintaining the widths thereof and bent inward in the extended region A₁. Accordingly, the end P of the end portions, 3 j and 3 k, come closer to the RF interconnection 5 in the joining portion 2 e. As described, the RF interconnection 5 shows slight impedance mismatching at the portion corresponding to the gap 2 f in the back surface 2 d, which increases the characteristic impedance of the RF interconnection 5. The arrangement of the end portions, 3 j and 3 k, bent closer to the RF interconnection 5, which may decrease the characteristic impedance of the RF interconnection 5, may compensate for the impedance mismatching at the gap.

Other end portions, 3 p and 3 n, for the electrodes, 3 a and 3 g, in the top surface 2 c, and portions, 3 h and 3 m, for the electrodes, 3 b and 3 f, in the top surface 2 c do not affect the electrical characteristic of the FPC board 1A because those electrodes, 3 a, 3 b, 3 f, and 3 g, are connected to DC signals and/or signals with low frequency components. Those end portions, 3 a, 3 b, 3 f, and 3 g, extending as keeping respective widths may effectively contribute to protect the RF interconnection 5 from being broken by bending along the line L or in the extended region A₁. Because those end portions, 3 h to 3 p, are bent inward in the extended region, a length intersected by the inner edge of the end portion A becomes longer, which further become effective against the bending of the FPC board 1A.

Third Embodiment

FIG. 4A is a plan view of a still another FPC board 11 according to the third embodiment of the present application. The embodiment shown in FIG. 4A has a feature that the FPC board 11 provides an overlay 12 to cover the mid body thereof and a most part of the extended region A₁. Other arrangements of the FPC board 11 are substantially same with those of the first embodiment shown in FIGS. 1A and 1B, and will omit the explanations thereof.

The overlay 12 provided in the FPC board 11, which may be made of resin, covers almost all portions of the interconnection 4 and the RF interconnection 5. The edge 12 a of the overlay 12 exits in the extended region A₁, exactly, in a side of the electrode region A₂ of the extended region A₁, which means that the overlay 12 covers greater part of the extended region A₁. The FPC board 11 of the embodiment further provides the extended portions, 3 q and 3 r, extending from the electrodes, 3 c and 3 e, in respective sides of the center electrode 3 d and electrically connected to the ground pattern 7 in the back surface, whose ends P exist in the joining portion 2 e crossing the extended region A₁. Assuming that no extended portions are provided in the electrodes, 3 c and 3 e, nor other DC electrodes, 3 b and 3 f, and/or, DC electrodes, 3 a and 3 b, the FPC board 11 is easily bent along the edge 12 a of the overlay 12 compared with a case that the FPC board 11 is bent along the boundary line L. The extended portions, 3 q and 3 r, may effectively protect the RF interconnection 5 from breakages by the bending of the FPC board 11 along the edge 12 a of the overlay 12 and along the boundary line L.

Fourth Embodiment

FIG. 4B is a plan view of a top surface of a still another embodiment of the FPC board 21 according to the present application. A feature of the FPC board 21 distinguishable from the aforementioned embodiment is that an overlay 22 provided in the FPC board 21 has a curved edge 22 a. Other arrangements including materials of the overlay 22 are same with those of the third embodiment. The overlay 12 of the present embodiment has a curved edge 22 a. Specifically, the overlay 22 extends in a center portion thereof toward the edge 2 a of the FPC board 21, while, retreats in respective side portions to expose the whole extended regions A₁ such that the edges 22 b of the overlay 22 locate on the joining portions 2 e. Thus, the overlay 22 may cover or protect the RF interconnection 5 further effectively.

Fifth Embodiment

FIG. 5A is a plan view of the top surface 2 c of an FPC board 31 according to still another embodiment of the present application. The FPC board 31 shown in FIG. 5A provides the interconnections, 4 and 5, and electrodes, 3 a to 3 g, substantially same as those of the second embodiment shown in FIG. 3A except that the electrodes, 3 b and 3 f, have no extended portions. Moreover, the FPC board 31 provides the overlay 12 that covers whole of the mid body and almost whole of the extended region A₁. That is, the edge 12 a of the overlay 12 comes closer to the electrode region A₂ or the boundary line L. Because the end portions, 3 j and 3 k, are bent inward, namely, closer to the RF interconnection 5, the end portions, 3 j and 3 k, may compensate for the impedance mismatching caused in the portion corresponding to the gap between the ground pattern 7 and the electrode 3 d in the back surface 2 d.

Sixth Embodiment

FIG. 5B is a plan view of still another FPC board 41 according to one embodiment of the present invention. The FPC board 41 provides the interconnections, 4 and 5, and the electrodes, 3 a to 3 g, same with those of the FPC board 31 shown in FIG. 5A and the overlay 22 same with those shown in FIG. 4B. That is, the overlay 22 has the edge 22 a with a convex center covering the RF interconnection 5 closer to the center electrode 3 d in the extended region A₁ and side portions retreated from the extended region A₁ to the joining portion 2 e to expose the interconnections 4. The ends P of the end portions, 3 j and 3 k, brought from respective extended portions, 3 q and 3 r, of the electrodes, 3 c and 3 e, put in the sides of the center electrode 3 d are bent inward closer to the RF interconnection 5 at the edge 22 a of the overlay 22. These bent end portions, 3 j and 3 k, may compensate for the impedance mismatching caused in the portion corresponding to the gap 2 f between the ground pattern 7 and the center electrode 3 d. Moreover, the bent end portions, 3 j and 3 k, intersect just at the edge 22 a of the overlay 22, namely, the edge 22 a of the overlay crosses the bent end portions, 3 j and 3 k, in a right angle, which may securely enhance the function to prevent the FPC board 41 from being bent at the edge 22 a of the overlay 22.

Seventh Embodiment

FIG. 6A is a plan view of another FPC board 51 according to the seventh embodiment of the present application. A feature of the present embodiment is that the FPC board 51 provides two RF interconnections 65 and the arrangement of the electrodes, 3 a to 3 g, in particular, the electrodes for the ground pattern.

The RF interconnections 65 extend straight from the edge 2 a and in parallel to the center axis AX. The outermost electrodes, 3 a and 3 g, are for DC signals and/or signals with low frequency components, which is same with those of the aforementioned embodiment, and connected to the interconnection 4. The electrodes, 3 b and 3 f, provided inside of the outermost ones, 3 a and 3 g, provide the extended portions, 63 q and 63 r, extending straight from the edge 2 a and bent inward to form the bent end portions, 63 h and 63 m. That is the ends P of the end portions, 63 h and 63 m, come closer to respective RF interconnection 65 in the joining portion 2 e and covered with the overlay 22. Similar to the former embodiments, the bent end portions, 63 h and 63 m, may compensate for the impedance mismatching of the RF interconnections 65 closer to the electrodes, 3 c and 3 e, where the ground pattern 7 in the back surface 2 d forms the gap against the electrodes, 3 c and 3 e. Because of the existence of the extended portions, 63 q and 63 r, in the extended region A₁, where the extended portions, 63 q and 63 r, are exposed from the overlay 22, may effectively protect the RF interconnections 65 from breakages due to the bending of the FPC board 51 along the boundary line L and/or the edge 22 a of the overlay 22.

The other FPC board 61 whose plan view is shown in FIG. 6B is a modification of the FPC board 51 shown in FIG. 6A. In the FPC board 61, the center electrode 3 d, which is connected to the ground pattern 7 in the back surface 2 d through the via hole 3 v, provides the extended portion 63 t in the extended region A₁. Moreover, the extended portion 63 t has two end portions 63 s in the joining portion 2 e. One of the end portions 63 s is bent toward the RF interconnection 65 connected to the electrode 3 c, and the other end portion 63 s is bent toward the other RF interconnection 65. Thus, these bend end portions 63 s and the other bent portions, 63 h and 63 m, shown in FIG. 6A may effectively adjust the characteristic impedance of the RF interconnection 65, in particular, the impedance mismatching in the portion corresponding to the root thereof closer to respective electrodes, 3 c and 3 e.

Although the present invention has been fully described in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom. 

What is claimed is:
 1. A flexible printed circuit (FPC) board soldered to a rigid circuit board, comprising: a top electrode provided in a top surface of one end of the FPC board; and a back electrode provided in a back surface of the one end of the FPC board, the back electrode being connected to the top electrode through a via hole and soldered to a pad provided in the rigid circuit board, wherein at least one of the top electrode and the back electrode extends an interconnection and another of the top electrode and the back electrode extends an extended portion terminated in the one end of the FPC board.
 2. The FPC board of claim 1, wherein the extended portion has a width substantially same with a width of the another of the top electrode and the back electrode.
 3. The FPC board of claim 1, wherein the one end includes an extended region and an electrode region, the electrode region including the top electrode and the back electrode, and wherein the extended portion of the another of the top electrode and the back electrode terminates in the extended region.
 4. The FPC board of claim 3, further comprising an overlay for covering the extended portion in the extended region and exposing the electrode region.
 5. The FPC board of claim 1, further comprising, another top electrode provided in the top surface of the one end of the FPC board, and another back electrode provided in the back surface of the one end of the FPC board, the another back electrode being connected to the another top electrode through a via hole and soldered to a pad provided in the rigid circuit board, wherein one of the another top electrode and the another back electrode provided extends another interconnection in one of the top surface and the back surface opposite to a surface where the interconnection is provided, and wherein another of the another top electrode and the another back electrode provided in the surface where the interconnection is provided extends another extended portion terminated in the one end of the FPC board.
 6. A flexible printed circuit (FPC) board connected to a rigid circuit board, comprising: a top RF electrode provided in one end of a top surface of the FPC board and a back RF electrode provided in the one end of a back surface of the FPC board, the top RF electrode extending an RF interconnection and being electrically connected to the back RF electrode through a via hole, the back RF electrode being soldered to the rigid circuit board; and a top ground electrode provided in the one end of the top surface and a back ground electrode provided in the one end of the back surface electrically connected to the top ground electrode through a via hole, the top ground electrode being next to the top RF electrode and the back ground electrode being next to the back RF electrode, the back ground electrode being soldered to the rigid circuit board, wherein the back ground electrode extends a ground pattern as forming a gap to the back RF electrode, and the top ground electrode extends an extended portion terminated in the one end of the FPC board.
 7. The FPC board of claim 6, wherein the extended portion has a width substantially equal to a width of the top ground electrode.
 8. The FPC board of claim 6, wherein the one end of the FPC board includes an extended region and an electrode region, the electrode region including the top RF electrode, the back RF electrode, the top ground electrode, and the back ground electrode, and wherein the extended portion of the top ground electrode terminates in the extended region of the FPC board.
 9. The FPC board of claim 8, further comprising an overlay for covering the extended portion of the top ground electrode and the RF interconnection in the extended region and exposing the electrode region.
 10. The FPC board of claim 6, wherein the top ground electrode further includes an end portion extending from the extended portion and terminated in the one end of the FPC board, the end portion being bent toward the RF interconnection in a portion corresponding to the gap in the back surface.
 11. The FPC board of claim 6, further comprising another top ground electrode and another back ground electrode electrically connected to the another top ground electrode through a via hole, the top ground electrode and the another top ground electrode sandwiching the top RF electrode, the back ground electrode and the another back ground electrode sandwiching the back RF ground, wherein the back ground electrode, the ground pattern, and the another back ground electrode surround the back RF electrode with a gap.
 12. The FPC board of claim 10, wherein the another top ground electrode provides an extended portion terminated in the one end of the FPC board.
 13. The FPC board of claim 12, wherein the another top ground electrode further provides an end portion extending from the extended portion of the another top ground electrode, and wherein the end portion of the another top ground electrode is bent toward the RF interconnection in a portion corresponding to the gap in the back surface.
 14. A flexible printed circuit (FPC) board soldered to a rigid circuit board, comprising: a plurality of RF interconnections each connected to respective top RF electrodes provided in one end of a top surface of the FPC board, the top RF electrodes being electrically connected to respective back RF electrodes provided in the one end of a back surface of the FPC board through via holes; a plurality of top ground electrodes and a plurality of back ground electrodes each electrically connected to top ground electrodes corresponding thereto through respective via holes, wherein the top RF electrodes and the top ground electrodes are arranged alternately, and the back RF electrodes and the back ground electrodes are alternately arranged, wherein the back RF electrodes and the back ground electrodes are soldered to the rigid circuit board, wherein the back ground electrodes are electrically connected through a ground pattern with gaps against the respective back RF electrodes, and wherein the top ground electrodes each has an extended portion with a width substantially equal to widths of the top ground electrodes and terminated in the one end of the FPC board.
 15. The FPC board of claim 14, wherein the top ground electrodes each includes an end portion extending from the extended portion, the end portion being bent toward the RF interconnection in a portion corresponding to the gap between the ground pattern and the back RF electrodes in the back surface of the FPC board.
 16. The FPC board of claim 15, wherein the one end includes an electrode region and an extended region, the electrode region including the top RF electrodes, the top ground electrodes, the back RF electrodes, and the back ground electrodes, the extended region terminating the end portions of the top ground electrodes.
 17. The FPC board of claim 16, further comprising an overlay for covering the RF interconnections and exposing the electrode region. 